[fsspec-aware #479] common: remote-aware load_constants_from_disk (read, priority 3)

[xroynard]

Sub-task of #479. Read side, transverse metadata layer — medium. Covers part ③ of the read-side analysis (issuecomment-5166383239).

Goal

Allow common/reader.py::load_constants_from_disk to read constants from a remote target.

Current state (verified on main)

• Backed by np.memmap over data.mmap (load_constants_from_disk, l.172).
• np.memmap requires a real seekable local file — passing an fsspec file handle fails with UnsupportedOperation: fileno (checked empirically).
• _materialize_memmaps already exists to copy memmaps into plain in-memory arrays, but the source still has to be a local file.

Constraint

Remote constants cannot be read in place. The data.mmap bytes must land on local disk (temp) first, then np.memmap + _materialize_memmaps as today.

Scope

• When the source is remote (fsspec), stage data.mmap (and companions) to a local temp file, memmap from there, materialize, then discard the temp.
• Keep the local fast path unchanged (no temp copy when already local).

Tests

• memory:// source: verify constants load correctly via the staging path.

Priority

★★ (3/5) — transverse blocker shared by all backends on the read side; needed before cgns remote read is meaningful.